1. Field of the Invention
The present invention relates to a process for the preparation of transparent and pigmented, heat-curable, weather-resistant polyurethane (PU) powder coating materials (powder coatings) of high reactivity and to the coatings obtained in accordance with this process.
2. Discussion of the Background
Heat-curable PU powder coatings based on partially or totally blocked polyisocyanates and hydroxyl-containing polymers whose softening point is above 40.degree. C. are part of the prior art and are described in numerous instances in the literature, such as in DE-A 21 05 777, DE-A 25 42 191, DE-A 27 35 497 (U.S. Pat. No. 4,246,380) , DE-A 28 42 641, DE-A 30 04 876, DE-A 30 39 824 or DE-A 31 28 743.
PU powder coatings contain essentially a hydroxyl-containing component and a polyisocyanate whose NCO groups are partially or completely masked with a blocking agent, so that the OH/NCO polyaddition reaction is unable to begin at temperatures below 140.degree. C. Only after heating to .gtoreq.150.degree. C. is it possible to bring about the crosslinking of the PU powder coatings to form a coating film within a practical period, with unblocking of the blocking agent and reaction of the OH groups with the NCO groups.
Of the host of blocking agents described in Houben-Weyl, Methoden der organischen Chemie (Methods of organic chemistry), Volume XIV/2, 4th edition, Georg Thieme Verlag, Stuttgart 1963, pages 61-70, only .epsilon.-caprolactam has become established in industry for the intended use of the blocked polyisocyanates in the PU powder coatings sector. To crosslink the coating, PU powder coatings based on .epsilon.-caprolactam-blocked polyisocyanates require curing temperatures between 170 and 200.degree. C.
There is, therefore, great interest in reducing the high curing temperatures in order to open up the field of powder technology to temperature-sensitive workpieces.
Another matter of interest is the reduction in the curing times, in order to enable an increase in the production rates, i.e. piece rates. In other words, economic as well as ecological factors are of importance.
Attempts have been made to achieve these objectives through the use of oxime-blocked (cyclo)aliphatic polyisocyanates. For example, oxime-blocked polyisocyanates and their use in PU powder coatings are described in DE-A 22 00 342, EP-A 0 432 257 and U.S. Pat. No. 3,857,818. EP-B 0 401 343 describes PU powders in which the hardener component is an acetone oxime-blocked trimethylolpropane/tetra-methylxylylene diisocyanate, while EP-B 0 409 745 specifies, as PU powder hardeners, 2,4-dimethyl-3-pentanone oxime-blocked and 2,6-dimethyl-4-heptanone oxime-blocked isocyanurates of isophorone diisocyanate (IPDI), of methylenebis-4,4'-cyclohexyl isocyanate and of m- and p-tetramethylxylylene diisocyanate.
EP-B 0 531 862 relates to a process for preparing powder coatings having a glass transition temperature of 20-80.degree. C. by mixing A) a polyol component, B) a ketone oxime-blocked polyisocyanate, C) a catalyst component consisting of at least one catalyst for the reaction between blocked NCO groups and hydroxyl groups, and, if desired, D) other auxiliaries and additives known from powder coating technology, the powder coatings being prepared by homogeneously dissolving components A, B, C and if used D, in an inert solvent or solvent mixture having a boiling point or boiling range which is between 50 and 150.degree. C. and then removing the solvent from the resulting solution.
Using oxime-blocked polyisocyanates it is indeed possible to prepare PU powder coatings whose curing temperatures are relatively low. However, a distinction must be made between transparent and pigmented coatings. The relatively high thermal instability of such PU powder coatings has an adverse effect: the coatings tend toward yellowing. A further disadvantage is their high susceptibility to the formation of defects ranging from pinholes to foaming, so that PU powder coatings containing oxime-blocked polyisocyanate are of limited usefulness and can be employed only for thin-film coating.
DE-A 28 12 252 describes 1,2,4-triazole-blocked polyisocyanates which are employed in PU powder coatings and are said to further improve the powder coating binders of the prior art. These compounds are 1,2,4-triazole-blocked diisocyanates and/or the urethane group-carrying polyisocyanates thereof.
DE-A 30 33 860 (EP 0 047 452) describes, on page 2, line 29 to page 3, line 6, that the blocked isocyanato-isocyanurate of hexamethylene diisocyanate (HDI) is unsuitable for use for PU powder coatings. An exception is constituted by the blocked isocyanato-isocyanurate of isophorone diisocyanate (IPDI). As EP 0 047 452 goes on to show, it is possible by cotrimerization of these two polyisocyanates (HDI/IPDI) to prepare products which in their blocked form are suitable (page 8, lines 16-21) for the PU powder coatings sector, although this is not demonstrated experimentally. Reference is made to the variability in the melting range as a function of the molar ratios of HDI/IPDI employed; increased solvent compatibility, low-temperature flexibility, etc., are specified (page 3, lines 19-21).
DE-A 33 22 718 describes blocked isocyanato-isocyanurates of 2-methylpentamethylene diisocyanate/2-ethylbutylene diisocyanate and IPDI cotrimers or mixtures.